Lasers are used, for example, for material processing and in semiconductor manufacturing, where the small feature size of today's circuit elements and interconnects demands very fine structural feature definition on the scale of tens of nanometers. Lasers are also employed in the production of optical gratings, such as volume gratings, holographic gratings, as well as in the distributed feedback (DFB) and distributed Bragg reflection (DBR) sections in diode lasers and optical fibers. The dimensions of the achievable structural features are related to the laser wavelength and to laser linewidth. Modern semiconductor fabs increasingly use wavelengths in the deep ultraviolet (DUV) and soft x-ray lithography for mask exposure. In addition, the definition of these small features also requires novel mask designs, such as phase masks, and the narrowest attainable laser linewidth, because a spectral distribution of the illuminating source would tend to wash out the desired features.
Suitable light sources operating in the UV and deep UV are excimer lasers, such as ArF, KrF excimer lasers, which have an emission wavelength of less than 250 nm and a FWHM (full width at half maximum) of about 300 pm.
In one conventional approach, an excimer laser cavity may be formed by an output coupler in form of a partially reflecting mirror and an echelle grating, reducing the linewidth of a KrF laser from 300 pm to about 0.8 pm. However, some applications, for example, submicron lithography (<250 nm) for integrated circuit fabrication requires a linewidth of about 0.5 pm or less. Alternatively, a double-pass configuration with a single echelle grating may be used for reducing the linewidth, whereby the laser radiation propagating in the cavity is incident on the grating twice with different polarization directions. However, this particular arrangement is not practical for generating the high laser output power required for high-throughput mask exposure due to the relatively low diffraction efficiency of conventional echelle gratings. The diffraction efficiency is typically about 60% per pass; therefore, the double pass loss from the gratings alone would be 0.6×0.6=36% which would result in a laser output power too low for applications in semiconductor manufacturing.
In another prior art approach for reducing the bandwidth, etalons are placed inside the laser cavity to filter out unwanted wavelength regions of the emission spectrum. However, etalons are susceptive to optical damage which makes them unsuitable for high power laser applications.
FIG. 4 illustrates yet another conventional approach similar to the laser cavity design disclosed in FIG. 6b of U.S. Pat. No. 6,795,473B1, the contents of which are incorporated by reference herein. The laser cavity 40 uses a traditional grism (prism-grating) 64 for wavelength dispersion and a separate prism pair 42 for beam expansion. The grism 64 transmits the principal order of the dispersion spectrum of the grating substantially in the direction of the incident beam, while outer portions of the spectral distribution of the incident beam are dispersed away from the beam path of the principal order. In this way, the grism 64 spectrally narrows the incident beam.
The prior art laser system 40 of FIG. 4 includes in addition to the grism 64 and the double-prism beam expander 42 a laser chamber 2, an partially reflecting output coupler 44 for an exiting output beam 41, and a reflective mirror 70. The grism 64 is essentially a prism with a grating surface 66 disposed on or formed on a major surface of the prism. The expanded beam from laser chamber 2 incident on the grating surface 66 of grism 64 and propagates collinearly with the incident beam through the rear prism surface 68 to mirror 70 where the beam is retroreflected back into the laser chamber 43 through the grism 64. The rear prism surface 68 may be AR coated at the lasing wavelength. The laser chamber 2 may be an excimer laser gain section, such as ArF or KrF. The output coupler 44 may have about 10% reflectivity at the lasing wavelength, although other reflectivity values may be selected. Not illustrated are apertures, beam scrambler, etalons or other prisms which may be required for optimal operation, but can be conventional and are not part of the invention.
While these prior art systems have proven somewhat useful in the past, a number of shortcomings have been identified. For example, the use of beam expanding prisms increases the number of optical surfaces in the laser cavity 40 which increases system complexity and cost. Further, prisms with a high beam expansion necessitate a large angle of incidence of the incident beam on the prism surface. Moreover, the light intensity transmitted across an air/dielectric interface at large angles of incidence disadvantageously depends strongly on the polarization direction of the light beam.
The development of integrated circuits with an ever increasing component density and decreasing feature size requires high power illumination sources operating at shorter wavelengths and having a narrow emission linewidth, while retaining high efficiency. Accordingly, there is an ongoing need for high-efficiency optical modules that are able to further narrow the optical linewidth of excimer lasers without sacrificing output power and electrical-to-optical conversion efficiency.